Many of the most basic physiological phenomena are mediated by specialized electrolytetransporting epithelia which, by forming unique absorbates and secretions, help create and maintain strikingly different environments on their mucosal and serosal sides. Elucidation of the underlying mechanisms of transport and selective membrane permeability in these epithelia represents one of the most fundamental and challenging problems for the biomedical researcher. Although past research efforts have focused primarily on numerous model systems in reabsorptive epithelia, the paucity of appropriate model systems in secretory epithelia has greatly limited our knowledge of ion transport in this class of transporting epithelia. This proposal describes a novel new model system to study transepithelial electrolyte secretion, the opercular epithelium from a teleost fish, which will provide ready access to mucosal and serosal compartments and is amenable to short-circuit current and ion flux studies. The objective of this proposal is to investigate mechanisms of transepithelial ion secretory transport using combined morphological and electrophysiological techniques such as ultrastructural cytochemical localization of Na, K-ATPase, fine structural visualization of electron opaque tracers, and measurement of key transepithelial electrical properties and ion fluxes. The proposed research will be focused on the following basic transport problems: a) the role of Na, K-ATPase in chloride secretion; b) the morphology of possible paracellular pathways for transepithelial ion permeation; c) the controversial mechanism of sodium transport in seawater teleosts; d) the roles of selected hormones in the mediation of chloride secretion. Study of this model system in expected to provide important insight into NaCl secretory mechanisms, and should render it useful as a model system in the areas of renal medicine and environmental toxicology.